1. Field of the Invention
This invention relates to an SOHC type internal combustion engine, and particularly to an improvement in an SOHC type internal combustion engine in which an ignition plug mounting hole which opens to a center portion of the top surface of the combustion chamber in the cylinder head and an ignition plug insertion hole communicated to the ignition plug mounting hole are provided, a cam shaft is held between a cam shaft receiving wall of a cylinder head and a cam shaft holder which is secured to the cam shaft receiving wall, and rocker arms which move in operative connection between the cam shaft and an intake valve and between the cam shaft and an exhaust valve are journaled to the cam shaft holder.
2. Description of the Prior Art
In such conventional internal combustion engine, a cylinder portion having the ignition plug insertion hole is formed integrally with or separately from the cylinder head, and the outer end of the cylinder portion is supported by a head cover so as to arrange an outlet of the insertion hole for a plug high power cord located on the exterior surface of the head cover. (For example: Japanese Utility Model Publication Kokai No. 87308/85)
However, in order to prevent interference of the cylinder portion with the rocker arms and a rocker arm shaft, the rocker arm and rocker arm shaft are required to have a special structure. A valve actuating system, therefore, becomes very expensive. The degree of freedom in design of the valve actuating system is limited because the positions of the above parts is restricted.